Lead-acid batteries contain lead plates that may be prepared by applying an aqueous paste of lead oxide (PbO) to a lead grid and then drying the grid. In some methods, for example continuous casting methods, the lead oxide is held in place by a pasting paper while the plate is dried. In other methods, such as strip casting methods, pasting paper is not needed.
Once dry, the plates are “formed” by applying an electrical charge to the plates while they are immersed in a 6 molar sulfuric acid solution, resulting in the creation of positive and negative plates. Newer production methods involve addition of expander materials (powdered sulfates) to the paste to produce negative plates, thereby eliminating the need to form the plates. In either case, a separator is then inserted between plates of opposite polarity, physically separating them. The separator's primary purpose is to prevent a short circuit due to particles bridging between plates of opposite charge. Once the separator is applied, oppositely paired plates are placed into a cell of the battery housing, electrolyte (dilute sulfuric acid) is added, and the cover is attached. The pasting paper (if present) typically degrades over time due to contact with the electrolyte.
A typical separator is a glass fiber mat. Although the mat must act as a barrier in the sense of preventing particle bridging between the plates, it should not interfere excessively with ion transfer in solution between the plates or reduced performance will result. The latter property encourages use of a relatively open, porous mat, but this may require the mat to be thicker to prevent particle bridging. Conventional separators have an overall thickness from 4-6 mm (0.157-0.236 in). This consumes additional volume in the battery and displaces electrolyte. This limits battery performance in terms of capacity and discharge rate, due to the lower amount of sulfuric acid available for ion exchange. Also, the trend towards smaller physical battery sizes makes these bulky conventional separators less than ideal.
In some batteries, the glass mat separator may be a so-called “absorptive glass mat” that fills essentially the entire space between plates, but that absorbs the sulfuric acid electrolyte such that there is essentially no free liquid acid. Such a battery may be used upside down or on its side without fear of acid spillage. Many of the same issues apply to absorptive glass mats as apply to traditional separators, i.e., that the desire to minimize thickness and still prevent particle bridging tend to be at cross purposes.
Thus, methods and devices for separating battery plates that address these or other current limitations of lead-acid batteries would be commercially beneficial.